Hysteriales

Hysteriaceae

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Introduction

Fungi classified in the Hysteriaceae Chevall. (Chevallier 1826) are defined by a specialized ascocarp termed the hysterothecium (Clements 1909). Hysterothecia are dense, persistent carbonaceous structures, distinctly navicular in outline, and bear a pronounced longitudinal slit running the length of the long axis of the fruitbody. Hysterothecia may be capable of opening partially to reveal a lenticular, disk-like hymenium or closing tightly in response to relative humidity, suggesting that some may be perennial, capable of spore discharge over prolonged periods (Lohman 1933). They can be immersed to erumpent or entirely superficial, solitary or gregarious, ellipsoid to greatly elongated, and are sometimes branched, triradiate or borne on a subiculum (Zogg 1962). In vertical section, hysterothecia are globose to obovoid, typically with a thick three layered peridium, composed of small pseudoparenchymatous cells, the outer layer heavily encrusted with pigment and often longitudinally striated on the surface, the middle layer lighter in pigmentation and the inner layer distinctly thin-walled, pallid and compressed (Barr 1987). The hamathecium is composed of persistent cellular pseudoparaphyses, often borne in a gel matrix, with tips darkened or branched at maturity to form an epithecium. Bitunicate asci are borne in a basal layer and at maturity are typically clavate to cylindric, bearing eight ascospores, overlapping biseriate, ranging from hyaline to dark brown, obovoid, clavate, ellipsoid or fusoid. Ascospores are highly diverse in septation and range from didymospores to phragmospores to dictyospores, at times surrounded by a gel coating, and often show bipolar asymmetry (Barr 1987). The family has been monographed by Zogg (1962). Detailed taxonomic keys and illustrations can be found on the web at http://www.eboehm.com/.

Taxonomic History

The genus Hysterium, the type genus of the family Hysteriaceae, is attributed to Tode (1784), who was the first to apply the name to a group of fungi bearing a pronounced longitudinal slit, for which he gave the common name “Venusschwämme”. Recognizing the transitional nature of the ascoma, Tode later (1791) stated: “Medium hoc genus inter Pezizas & Lichenes”. Due to the seemingly transitional nature of the hysterothecium, neither fully open nor closed, hysteriaceous fungi have been placed in the discomycetes and pyrenomycetes about equally by various mycologists throughout the 19th Century (Bisby 1923). In his Systema Mycologicum, Fries (1823) initially considered hysteriaceous fungi to belong to the pyrenomycetes and placed them in the order Phacidiacei, but later (1835) placed them in his new class discomycetes, stating: “Transitum sistunt ad Discomycetes, sed discum verum non monstrant.” Ellis and Everhart (1892), in their North American Pyrenomycetes, tentatively included the Hysteriaceae, but stated that they had not at first intended to do so due to the transitional nature of the hysterothecium. Duby (1862) considered hysteriaceous fungi to belong to the pyrenomycetes and proposed two sections: the Hystériées to include Hysterium, Glonium, and Actidium Fr. among others, and the Lophiées to accommodate Ostreichnion Duby, Mytilinidion Duby and Lophium Fr. Although Duby’s (1862) method of classification, based on dehiscent versus nondehiscent asci, was not followed by subsequent workers, he was the first to propose dividing hysteriaceous fungi into what was later to become two distinct families. However, one hundred years would pass before this distinction was fully recognized (Zogg 1962).

Early authors did not follow Duby (1862) and instead classified fungi currently recognized as belonging to the Mytilinidiaceae within the family Hysteriaceae due to perceived similarities in ascocarp morphology, specifically its means of longitudinal dehiscence (Fries 1823; Ellis and Everhart 1892; De Notaris 1847; Massee 1895; Rehm 1896; Saccardo 1883; von Honel 1918). Modern authors have likewise included mytilinidiaceous fungi within the Hysteriaceae, placing the family in the Pseudosphaeriales (Nannfeldt 1932; Gäumann 1949), the Dothiorales (Müller & von Arx 1950; von Arx & Müller, 1954), the Dothideales (von Arx & Müller 1975) and in a separate order the Hysteriales, closely related to the Pleosporales (Miller 1949; Luttrell 1955). Luttrell (1953) studied ascomal ontogeny and hamathicial development in Glonium stellatum Mühlenb.:Fr. and concluded that the Hysteriaceae possess the pseudoparaphysate Pleospora-type centrum, in which cellular, septate pseudoparaphyses grow downwards from the cavity roof, initially anchored at both ends, and occupy the locule prior to the formation of asci (Luttrell 1951). The Hysteriales were placed in the subclass Loculoascomycetes by Luttrell (1955), due to the presence of bitunicate asci, corresponding to the Ascoloculares first proposed by Nannfeldt (1932). Zogg (1962) acknowledged the heterogeneity of the classical Hysteriales and, following Duby (1862), divided hysteriaceous fungi into two families, namely the Hysteriaceae to accommodate sessile, thick walled hysterothecia & the Mytilinidiaceae to accommodate modified hysterothecia, generally erect and possessing thin walls. Luttrell (1973) held a wide concept of the Hysteriales and did not recognize the family Lophiaceae, instead proposing a subfamily within the Hysteriaceae to accommodate mytilinidiaceous forms. Barr (1979) however maintained the two family distinction. The Mytilinidiaceae was placed in the Melanommatales, based on a thin-walled peridium of scleroparenchymatous cells enclosing a hamathecium of narrow trabeculate pseudoparaphyses, asci borne in a peripheral layer and with ascospores typically showing bipolar symmetry (Barr 1987, 1990). Barr (1983) eventually abandoned the Hysteriales and placed the Hysteriaceae within the Pleosporales due to the presence of cellular pseudoparaphyses, asci borne in a basal rather than peripheral layer and ascospores typically showing bipolar asymmetry. Kirk et al. (2001) maintained both the Hysteriaceae and the Mytilinidiaceae in the Hysteriales, but Eriksson (2006) removed the Mytilinidiaceae from the Hysteriales and considered it as Dothideomycetes et Chaetothyriomycetes incertae sedis, leaving the Hysteriaceae as the sole family in the Hysteriales.

Recently, Schoch et al. (2006), using a multigene phylogeny of the Dothideomycetes, based on the nuSSU and nuLSU, the transcription elongation factor (TEF1) and the RNA polymerase II second largest subunit (RPB2), provided evidence indicating that hysteriaceous fungi do not form a monophyletic group. Data from this preliminary analysis concluded that the Hysteriaceae should be regarded as Dothideomycetes incertae sedis. More recently, Boehm, Schoch & Spatafora (2008, unpublished), employing a much wider taxon sampling strategy, and using the same four genes, investigated whether morphological features historically used in the delineation of higher taxa in the Hysteriaceae were phylogenetically informative in the context of sequence-based phylogenies. It was found that the Hysteriaceae resides within the subclass Pleosporomycetidae, adjacent to the Pleosporales, and constitutes its own order, the Hysteriales.

Kantvilas G, Coppins BJ, 1997. Melaspilea circumserpens Nyl. rediscovered and referred to Glonium, with discussion on the provenance of some of Robert Brown’s lichen specimens. Lichenologist 29: 525-531.

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Hysteriales. Hysteriaceae.
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